It is frequently desired to separate organic acids from feedstocks containing them. The term "feedstocks" as used herein is intended to denote aqueous media generally, whether solutions or suspensions, which have an undesired content of organic acids. Such media include, for example, process liquids and waste streams from the food industry, or from chemical production plants utilizing synthetic or extractive methods, or biotechnological methods. In the known art, the following techniques have been used to effect such separations, namely, ion exchange, solvent extraction, absorption with selective resins, extraction with supercritical gases, and the action of membranes.
When effecting such separations, it is often desired to otherwise maintain the composition of the aqueous feedstocks substantially unchanged. By way of example, it may be noted that absorptive processes in which citrus juices were contacted with ion exchange resins, have been used to remove citric acid and the bitterness due to limonin, therefrom (Johnsson and Chaundler, CSIRO Food Res. Q. 1985: 4525-32); most of the absorbents used in these processes did not absorb juice sugars, had no adverse effect on the fruit character of the juices and in particular did not introduce objectionable off flavors. The main problem observed during the processing of orange juice related to the slight loss of vitamins, minerals and amino acids; see Assar, Minute Maid Reduced Acid, FCOJ, 19th annual short course for the food industry, 1979. Moreover, adsorption processes require regeneration of the absorbent, resulting in the use of chemicals, consumption of energy and a waste disposal problem for the regenerating chemicals.
Selective separations using membranes offer considerable advantages over other known separation processes. Thus, they can readily be adapted to a commercial scale and to continuous operation, they do not require the use of regenerating chemicals and offer substantial economic advantages. For these reasons, conventional separation techniques are being increasingly replaced by techniques utilizing selective membranes. Such techniques include reverse osmosis (RO), ultrafiltration (UF) and microfiltration (MF), all of which are pressure driven, and electrodialysis (ED), which as the name implies is electrically driven.
In RO, UF and MF, a liquid stream containing soluble and suspended matter is circulated parallel to the membrane surface (cross flow) and pressurized simultaneously. Water and some soluble substances are transported across the membrane, while the retained soluble and suspended substances are concentrated. These processes differ in the dimensions of the membrane pores: in UF membranes they may range from 1.5 to 100 nanometers, in MF membranes from 0.05 microns to 10 microns, while in RO membranes they may range from 0.1 to 1.0 nanometers. Thus UF is basically a sieving process--the small molecules are responsible for the established osmotic pressure but are not retained by the membrane, the applied hydraulic pressures are thus not high; they may be of the order of about 5 bars, as compared with the higher pressures of say 10 to 100 bars in the case of RO.
Diafiltration (DF) is a modification of pressure driven processes (mainly UF and MF) in which water is added to the feed, to maintain its volume constant. As filtration proceeds, the components are effectively washed out from the feed and pass through the membrane, the rate of adding water to the feed equals the rate of permeate removal. The diluted permeate stream is regarded as waste and is often discarded.
In ED, a feed containing ionized species is circulated in a stack of alternating cation and anion exchange membranes under an applied electric field, so that the ionized species are transported from the feed into the adjacent compartments. Ion exchange membranes have pores of the order of inorganic and relatively small organic ions; inorganic ions in particular can be effectively removed by this process. In the case of small organic ions having a molecular weight below 200, these can also be readily removed by means of ED in the absence of fouling agents, but if large organic ions above 400 daltons are present they plug the membrane pores with the result that the process becomes very inefficient. Thus, there are relatively few successful commercial applications of ED where organic ions are involved.
As indicated above, unit separation processes such as UF, MF and RO are being increasingly applied in the food industry, e.g. for concentrating liquid products. Such processes are especially beneficial for products which would be adversely affected by high temperatures, and are also energy efficient. In this connection, reference may be made, for example, to "Water and ion flow-through imperfect osmotic membranes", Breton E. J., Dissertation Abstr. 18: 822 (1958). Applications of UF and RO for concentrating liquid food products were initiated in the dairy industry in the 1960s. Marshall et al in Food Technol. 22: 969 (1968), studied the concentration of cottage cheese whey solids by RO as an alternative to whey disposal. Industrial scale applications of UF and RO in the dairy industry are summarized by Glover, National Institute for Dairying, Reading, England, 1985. Other studies described UF and RO concentrating techniques for concentrating various liquid food products such as maple syrup, egg white, fruit and vegetable juices, and plant pigments such as anthocyanins. In general, these researches concluded that such techniques, modified as necessary, could be applied to the food industry.
A combined UF and RO process is also known in the literature for the preparation of purified beet color extracts, which were separated from soluble solids originally present in the feed. The juices were prefiltered, enzymated and subjected to a two stage UF process, using in sequence UF membranes with 20,000 and 6000 molecular weight cutoff, in order to remove soluble materials of high molecular weight. Such materials, if present in a stream contacting RO membranes, would plug the pores and therefore reduce the throughput rates and recovery of the natural color. In order to recover as much as possible of the natural color in the permeate of each UF step, the feed was repeatedly diluted and washed out with fresh water. In the subsequent RO step, the solution was subjected to combined concentration and purification, the RO membrane being endowed with high retention of the color, while allowing inorganic salts, sugars and beet taste components to permeate. A highly concentrated beet color product with improved sensory properties was thus obtained. In this example, UF and RO membranes were used to fractionate a selected component from an aqueous stream; all fractions except the color were of minor value and could be discarded. It is to be noted that large quantities of rinsing water were used for extracting the color while removing high molecular weight contaminants in the UF steps, without losing the color when low molecular weight contaminants were being removed in the subsequent RO step. In such a process, the high dilution of the permeate solubles means that their recovery is not usually economic and they are lost in the discarded waste stream.
UF membranes have also been used for the preparation of protein concentrates having reduced lactose content, from skim milk. Simple UF concentrates the protein, but leaves a product containing some lactose and salts, the content of which may be reduced by DF (see Ultrafiltration and Reverse Osmosis for the Dairy Industry, National institute for Dairy Research, Reading, England, 1985, p. 100). In the DF step, water is added at a rate which keeps protein concentration constant; it is undesirable to add water at a greater rate, because by keeping the lactose concentration high in the feed, it is removed at a relatively faster rate. In this example also, the permeating substances are of low value and can be discarded.
In many cases, it is desired to remove selected substances from aqueous stream, while imparting minimal changes to the remaining composition of the feed. However, the use of a washing/DF step in conjunction with selective, pressure driven process utilizing membranes has the drawbacks that large quantities of rinse water are required and that almost inevitably large quantities of feed components are lost through the selective membranes, and are not economically recoverable. Also, membranes with sharply defined selectivities are not available for many applications, while the use of less selective membranes tends to accentuate the losses of valuable solids.
Ionized organic substances can generally be removed from feeds by ED, of which examples are as follows:
(1) Smith et al (R & D Associates Convenience Food Conference, Philadelphia, 1964) reported the removal of citric acid from citrus juices and improvement of their organoleptic properties by ED; other workers reported similarly on the removal of a variety of organic acids from apple and citrus juices and from wines.
(2) Malic acid effluent containing less than 10% malic acid alone or in admixture with maleic and fumaric acids, when treated by ED, produces two streams, one of about 30% acid and the other less than about 0.3% (U.S. Pat. No. 3,752,749). In this case, the separation and concentration of acids is non-selective and the feed is relatively clean, being free of fouling agents which can clog the membranes and thus interfere with the separation process.
(3) The separation by ED of organic amino acid esters from admixtures with amphoteric amino acids is described in Israel Patent Nos. 52686 and 52687. In this case also, the feed is free of fouling agents.
Many industrial feed or waste streams may contain suspended or soluble substances which either adhere to the membrane pores or accumulate therein and thereby prevent efficient ion transfer. The problem of ED membrane fouling arises, for example, in demineralization of cheese whey (fouling by proteins), deacidification of citrus juices (fouling by high MW pectic substances) and demineralization of sugar molasses (fouling by proteins). Further, fouling may arise due to the presence of low and medium size molecules which adsorb strongly on and in the ED membranes; e.g., whereas organic anions MW&lt;about 200 daltons are readily transported across anion exchange membranes, larger organic ions MW&gt;about 450 are usually very problematic. Thus, the biodegradation product humic acid is present in most natural waters as colloidal negatively charged matter, and during ED such ions accumulate progressively on and in the membrane, and the electrical resistance of the ED stack is raised to the point where operation becomes uneconomical.
Various attempts have been made to overcome such difficulties, which would otherwise tend to delay or even prevent the penetration of ED technology into industrial and waste treatment fields.
Thus, in U.S. Pat. No. 4,554,076 there is described a method of modifying the surface of anion exchange membranes by coating with oriented layers of amphiphilic surface active molecules (having one polar and one hydrophilic end). This method does not have appeared to have been used on an industrial scale. Japanese Patent No. 1014234 (1980) describes the difficulties encountered in attempting to desalt sugar molasses streams, due to fouling of the (otherwise efficient) anion exchange membranes, and a solution of the problem by replacing the latter with a neutral porous layer of PVA, which reduces the efficiency of the process.
It is also known to pretreat the feeds to remove fouling substances, e.g. by use of UF to remove proteins and other contaminants prior to an ED step, but such processes have the following drawbacks:
(a) High extraction recoveries require high volumetric efficiencies of the UF/MF/RO step, which in turn means operation at high concentrations of fouling substances, i.e. low flux.
(b) If it is attempted to increase recovery without excessive concentration of the foulants, diafiltration or washing steps may be introduced. However, this leads to a diluted permeate, so that the economy of the subsequent ED step is impaired.